Fix typing errors after mypy update

Author: nicoddemus

src/pluggy/_hooks.py

@@ -501,7 +501,7 @@ def __init__(self, orig: _HookCaller, remove_plugins: AbstractSet[_Plugin]) -> N
         self._hookexec = orig._hookexec  # type: ignore[misc]
 
     @property  # type: ignore[misc]
-    def _hookimpls(self) -> List["HookImpl"]:  # type: ignore[override]
+    def _hookimpls(self) -> List["HookImpl"]:
         return [
             impl
             for impl in self._orig._hookimpls

src/pluggy/_manager.py

@@ -191,7 +191,7 @@ def parse_hookimpl_opts(
             res = {}  # type: ignore[assignment]
         if res is not None and not isinstance(res, dict):
             # false positive
-            res = None
+            res = None  # type:ignore[unreachable]
         return res
 
     def unregister(

testing/test_pluginmanager.py

@@ -2,6 +2,7 @@
 ``PluginManager`` unit and public API testing.
 """
 import pytest
+import sys
 from typing import Any, List
 
 from pluggy import (
@@ -11,7 +12,11 @@
     PluginManager,
     PluginValidationError,
 )
-from pluggy._manager import importlib_metadata
+
+if sys.version_info >= (3, 8):
+    from importlib import metadata as importlib_metadata
+else:
+    import importlib_metadata
 
 
 hookspec = HookspecMarker("example")